In direct-conversion or low-intermediate-frequency radio architectures, such as are commonly used for Wideband Code Division Multiplex Access (WCDMA) band and multi-band cellular telephones, stringent requirements are placed on linearity in the receive signal path. Specifically, the receive signal path should desirably generate very low second-order and third-order intermodulation products while still achieving low noise figure and consuming low power. High linearity, low noise, and low power are conflicting design requirements, and it is difficult to simultaneously achieve all three objectives.
Numerous conventional efforts deal with the issue of mixer linearity, because it is such an important issue in radio design. The use of emitter degeneration is common. Two-stage designs are also commonly used to achieve a better tradeoff between noise and distortion. Significant prior efforts are described in the following patents and publications: U.S. Pat. No. 4,485,359 to Talbot et. al. (Nov. 27, 1984), “Amplitude Modulator Having Substantially Zero Modulation Distortion”; U.S. Pat. No. 5,548,840 to Heck (Aug. 20, 1996), “Balanced Mixer Circuit With Improved Linearity”; U.S. Pat. No. 5,497,123 to Main et. al. (Mar. 5, 1996), “Amplifier Circuit Having High Linearity For Cancelling Third Harmonic Distortion”; U.S. Pat. No. 5,886,547 to Durec et. al. (Mar. 23, 1999), “Circuit and Method of Controlling Mixer Linearity”; B. Gilbert, “The Micromixer: A Highly Linear Variant of the Gilbert Mixer Using a Bisymmetric Class-AB Input Stage”, IEEE JSSC, Vol. 32, No. 9, September 1997, p. 1412, Analog Devices; and C. Trask, “A New Low-IMD Mixer”, Electronics World, Vol. 106, No. 1773, September 2000, p. 680.
The patents to Talbot et al. and Heck both involve the application of series feedback to improve the linearity of an emitter degenerated double-balanced mixer. The feedback circuits disclosed in both patents s do not provide a controlled impedance single-ended input. In fact, the circuits in these patents would appear to increase the input impedance.
The patents to Main et al. and Durec et al., as well as the Gilbert article, involve the application of series feedback to an unusual bisymmetric, folded common-base input stage. The primary disadvantage of these circuits is high noise figure, and there is also no easy way to apply variable gain.
Trask appears to disclose the application of shunt feedback to a double-balanced mixer circuit. However, Trask applies shunt feedback from the mixer output back to the mixer input. Since the action of the mixer transforms the signal frequency, the feedback to the input is at a different frequency than the input signal.
In view of the foregoing, a need has been recognized in connection with overcoming the disadvantages and shortcomings presented by conventional efforts.